Power factor measures the ratio of average power to the apparent power in an electrical load. Power factor ranges from a value of 0 (where the impedance of the load is purely reactive) to 1 (for a purely resistive load). In practice, the power factor of electrical devices ranges somewhere between 0 and 1, and the closer this value is to unity, the more efficiently energy is consumed by the device and the less power is wasted. Therefore, for consumers of electricity that employ highly reactive loads (e.g. electrical induction motors), it is desirable that steps be taken to adjust the power factor of their apparent load to improve performance and avoid wasting enormous amounts of power. For example, a mill that consumes 100 kW from a 220-V line with a power factor of 0.85 will require 118 kW of apparent power supplied. But if the power factor is improved to 0.95, the apparent power supplied drops to 105.3 kW. Many utility companies require such consumers to take affirmative steps to adjust power factor.
Large factories are not the only environments to benefit from improvement in power factor. AC motors are present in many different electrical appliances and equipment from compressors to elevators, and since they are usually inductive in their input impedance, they often present a less than desirable power factor rating, especially under light load conditions or during certain periods of load variance. To improve the power factor in AC motors, controllers have been developed and are generally known in the art. Examples include U.S. Pat. No. 4,459,528, entitled “Phase Detector for Three-Phase Power Factor Controller” (Nola '528); U.S. Pat. No. 4,266,177, entitled “Power Factor Control System for AC Induction Motors” (Nola '177); and U.S. Pat. No. 5,821,726 entitled “Balanced and Synchronized Phase Detector for an AC Induction Motor Controller” (Anderson); all of which are hereby incorporated by reference.
In general, the power factor mitigation approach taken by many AC motor controllers is accomplished by sensing the phase difference between the current and voltage phasors and then using a controller to adjust the actuation of thyristors in each AC motor phase to attempt to reduce the voltage and current phase lag. In an ideal implementation, if the phase between the current and voltage phasors can be brought to zero, the load looks resistive to the power supply, and therefore, the power factor would approach unity. While unity power factor is not entirely practically achievable, small improvements in power factor can make substantial differences in power consumption.
Many different approaches to improving power factor in electrical motors have been developed over the years. For instance, U.S. Pat. No. 4,052,648 (entitled “Power Factor Control System for AC Induction Motors” (Nola '648)) describes a power reduction system for less than fully loaded induction motors, which is hereby incorporated by reference. The phase angle between current and voltage (motor power factor) is controlled. In the Nola '648 system, the motor power factor is controlled as a function of the difference between a commanded power factor signal and the operating power factor through control of thyristors connected to the motor.
U.S. Pat. No. 4,266,177, entitled “Power Factor Control System for AC Induction Motors” (Nola '177), is a system adapted to respond to conditions where motor loads are abruptly changed (e.g., by increasing the speed of full motor voltage turn-on). In the Nola '177 system, a phase comparison is made by combining the voltage and current derived square wave signals and generating a series of pulses equal in time width to the phase angle between motor current and voltage. The variable width of the pulse is changed to a variable-amplitude DC signal whose amplitude is proportional to the phase difference. The phase difference signal is compared with a command voltage signal representing a desired minimum power factor of operation. The resulting difference signal, a circuit error signal, is then used to control the on time of a triac in series with the winding of the induction motor to maintain motor operation at the selected power factor. This has the effect of significantly reducing the power input to a less than fully loaded motor.
Inherent in changing these pulses into a DC signal is the necessity to filter the pulses to produce a smooth DC signal. This is accomplished, for instance, by using an analog integrator. Analog integrators, by their very nature, can create a time lag between the actual change in phase angle and the change the controller sees. This lag can be quite significant compared to the motor's ability to respond to a change in load. As a result, designs such as those shown in U.S. Pat. No. 4,266,177 may require additional circuitry to cancel this time lag during periods where the motor's load suddenly increases.
U.S. Pat. No. 5,821,726 addresses the response to large increases in motor load. The speed of response to a change from lightly loaded to fully load conditions could be inadequate to prevent motor stalling or vibrations, especially when the minimum power factor command setting is relatively high. For example, although the Nola system(s) responded fairly quickly, further improvements could be made to respond to large abrupt load changes such as those in oil well pumps, motor generator sets, stamping machines, refrigerator compressors, and the like.
The filtering of the pulse train to create a smooth DC signal can compromise the response of the controller. High efficiency motors connected to controllers may also enter into undesirable periods of excessive vibration due to the controller's inability to respond to these high efficiency motors.
It would be desirable to provide a fast responding controller that has the capability to improve the control of the phase lag in induction motors and hence the amount of energy saved. It would also be desirable to provide a controller that is capable of working with a broad variety of electrical appliances that contain induction motors thereby improving power factor and start-up characteristics. It would also be desirable to provide a power factor improving controller that is programmable and may be customized to particular loads and operating conditions.